1. Field of the Invention
The present invention relates to a structure of panel in flat-type CRT (Cathode Ray Tube), and more particularly, to a structure of panel in flat-type CRT, which is capable of improving implosion-proof properties of a flat-type CRT by effectively reducing the advance of cracks caused by an external shock and scattering of fragments resulting from the shock.
2. Description of the Related Art
In general, as shown in FIG. 1, a flat-type CRT (Cathode Ray Tube) includes: a panel 1; a shadow mask 3 fixed on a rear surface of the panel 1 such that a tension is applied to the shadow mask 3 and having a plurality of apertures of round or slot type for selecting colors of an electron beam 6; a magnetic shield 7 fixed on the inside of the panel 1 to prevent the path of electron beam 6 from being changed by an earth magnetic field or a leakage magnetic field; a funnel 2 fixed on the panel 1 by a frit glass and having a neck part formed integrally at a rear portion; an electric gun (not shown) inserted and sealed in the neck part of the funnel 2 for emitting the electron beam 6 of three colors, i.e., R, G and B colors; and a deflection yoke 5 mounted to wrap the external circumference of the neck part for deflecting the electron beam 6.
Meanwhile, because the inside of the flat-type CRT may be easily damaged due to an external shock (being in a high vacuum condition), the panel 1 is designed to withstand atmospheric pressure.
Moreover, the panel 1 is divided into a face part 1a and a skirt part 1b. The skirt part 1b has a safety band 8 mounted to disperse stress applied to the flat-type CRT due to the high vacuum condition and to secure the shock resistance capacity.
When the flat-type CRT is operated, the electron beam 6 of the electric gun mounted in the neck part of the funnel 2 strikes a luminescence surface 4 formed on an inner surface of the panel by anode voltage applied to the flat-type CRT. The electron beam 6 is deflected in all directions by the deflection yoke 5 before reaching the luminescence surface 4, and then it reaches the luminescence surface 4.
At this time, the neck part has magnets 9 of bipolarity, tetrapolarity and hexapolarity at a rear portion for correcting an advance orbit in order for the electron beam 6 to strike a prescribed fluorescence body, thereby preventing staining that affects color purity.
Referring to FIG. 2a, a structure of the panel of the flat-type CRT will be described hereinafter in more detail.
In general, the panel of the flat-type CRT has an outer surface (in the form of a plane) and a curved inner surface having a prescribed curvature. As shown in FIG. 2a, the panel 1 is the thinnest in a center face thickness (hereinafter, called as a CFT) and becomes gradually thicker toward the outer circumference.
The outer circumference of the panel 1 has a discontinuous part generated during a molding process of the panel. The discontinuous part is a mold match line (hereinafter, called as a MML) and is the same form which a belt is bound around the outer circumference of a panel thereof.
At this time, a size of a mold match height (hereinafter, called as a MMH), which is a height from the MML to a front surface of the panel 1, is larger than that of the CFT of the panel 1.
Particularly, an opposite angle portion thickness (OAPT) of the panel 1 is designed to be thick 160% or more, compared with the CFT.
A height from an end of the skirt part 1b of the panel 1 to a front surface of the face part 1a is designated as an overall height (hereinafter, called as an OAH).
A manufacturing process of the panel of the conventional flat-type CRT will be described as follows.
In general, as shown in FIG. 2a, the outer circumference of the panel 1 has prescribed angles xcex81 and xcex82 formed toward the face part 1a and the skirt part 1b respectively centering around the MML. Thus, in consideration of a slip of the mold, if only one external mold is used, the molding cannot be performed.
Therefore, as shown in FIG. 2b, one internal mold 10 and two external molds 11a and 11b are combined and used.
Here, the external molds are divided into an upper external mold 11a and a lower external mold 11b. 
Therefore, when the panel 1 is molded, the upper and lower external molds 11a and 11b are matched to form an external form of the panel 1. After a glass material of a prescribed amount is inserted into the external molds 11a and 11b, the internal mold 10 (to form the inner surface of the panel 1) is lowered to a position where a prescribed interval between the internal mold 10 and the external molds 11a and 11b is maintained. The internal mold 10 is raised up after a predetermined period of time is passed.
At this time, the panel 1 must be formed to have a thickness sufficient to endure a predetermined vacuum pressure after the CRT is finished. The interval between the external molds 11a and 11b and the internal mold 10 must be set to have different intervals according to the standard of the panel 1.
That is, the CFT of the panel 1 is determined by the interval between the center of the external molds 11a and 11b and the center of the internal mold 10.
Because the cathode ray tube manufactured by the above method is made of the glass material and the inside of the cathode ray tube is in a vacuum condition, there is a danger of accidents and scattering of the fragments if cracks or implosion occurs due to an external shock. The safety band 8 made of a metal material is attached to the skirt part 1b of the panel 1 to prevent such danger.
The reason that the safety band 8 is attached to the skirt part 1b of the panel 1 is that the greatest tension stress caused by the vacuum is at the skirt part 1b and the scattering of the glass fragments is generated in the skirt part 1b as well.
Therefore, the safety band 8 is contacted to the skirt part 1b of the panel 1 apply sufficient tension to the safety band 8.
At this time, the tension of the safety band 8 must be adequate not only for the skirt part 1b but also for the face part 1a of the panel 1.
Conventionally, the safety band 8, which is bent to correspond with the outer angles of panel 1 of lower portion of MML xcex81 and with the outer angles of panel 1 of upper portion of MML xcex82, is used to transfer the sufficient tension to the face part 1a of the panel 1.
However, there is a problem in that the tension of the safety band 8 is not applied sufficiently to the face part 1a in spite of the bent structure of the safety band 8.
That is, as shown in the drawing, based on the MML, because a circumference of the skirt part 1b located at the lower portion of the MML is larger than that of the face part 1a located at the upper portion of the MML, when the safety band 8 wound in a heat expansion state is contracted while cooled, stronger tension is applied to the skirt part 1b, which has the outer circumference larger than that of the face part 1a, compared to the face part 1a. 
In the conventional panel 1, as described above, because the tension is not sufficiently applied to the face part 1a of the panel 1, the crack generated by shock easily advances to the inside of the panel 1 as shown in FIG. 4, and thereby the crack may be generated throughout the face part 1a of the panel 1.
That is, in the structure of the conventional panel 1, the MML located at the lower portion of the CFT does not effectively prevent the advance of a crack toward the inside of the panel, and thereby there is a limitation in that the panel 1 has stable implosion-proof properties.
Furthermore, to use the safety band 8 of the bent structure, equipment for bending a straight band must be prepared, and thus additional expenses for preparing the equipment are required. Moreover, a recovery rate of the product is lowered in comparison with the straight band 8, and thus manufacturing costs are increased.
The reason that the safety band 8 of the bent structure is used in spite of the above disadvantages is to solve a problem of the straight safety band in that the safety band is contacted to only the skirt part 1b located at the lower portion of the MML of the panel 1 and thereby the tension is concentrated on the skirt part 1b. 
That is, in a case of using the straight safety band on the panel 1, because the angle xcex82 formed toward the face part 1a located at the upper portion based on the MML is still larger than the angle xcex81 formed toward the skirt part 1b located at the lower portion based on the MML, the tension of the safety band is concentrated on the skirt part 1b, and thereby the crack of the face part 1a advancing by the external shock is not reduced effectively and the scattering of the fragments due to shock is not effectively prevented.
Meanwhile, it is advantageous to reduce the MMH to apply stronger tension to the face part 1a of the panel 1 and to secure the stable proof-implosion-proof properties.
However, if the OAH is left as is and only the MMH is reduced, the length of the skirt part 1b is increased. Thus, in case forming the panel using the mold, when the upper external mold 11a is separated, scratches or other deformation may occur in the skirt part 1b. 
Furthermore, in case that the OAH is left as is and only the MMH is reduced, if the upper external mold 11a is separated in a state in which the glass material is not sufficiently cooled, the skirt part 1b, which is not hardened completely after the molding, may be transformed due to its own weight. Moreover, even though the transformation due to the weight of the skirt part 1b does not occur, the skirt part 1b may be transformed by being shaken by external influences, e.g., vibration of a conveyer, when the skirt part 1b is transferred to the next step.
Meanwhile, the CRT, which has the inside of a vacuum condition, must effectively recover from a depression of the panel 1 due to the vacuum condition by the reinforcement of the safety band. However, if the length of the skirt part 1b of the panel 1 is short, the safety band cannot secure a sufficient width, and thereby the CRT cannot recover the panel 1 to its original condition.
Moreover, if the length of the skirt part 1b of the panel 1 is short, the tension stress against glass products is applied to a conjunction part between the panel 1 and the funnel 1. To solve the above problem, the OAH must be long.
On the contrary, if the length of the skirt part 1b of the panel 1 is too long, the skirt part 1b of the panel 1 becomes too thin to secure available picture area in the inside of the panel 1. In this case, a relatively high stress is applied to a connection part between the face part 1a and the skirt part 1b. 
In brief, in the conventional panel structure, since the MMH is larger than the CFT, sufficient tension is not applied to the face part 1a, and thus it is difficult to obtain a stable vacuum intensity and to effectively reduce the advance of a crack. Furthermore, additional equipment expenses for bending the safety band are required.
Therefore, to solve the above problems and to secure the stable vacuum intensity and the implosion-proof properties of the panel 1, a demand of the optimization of the relationship among the MMH, the CFT and the OAH is on the rise.
It is, therefore, an object of the present invention to provide a panel in flat-type CRT (Cathode Ray Tube), which is capable of securing a stable vacuum intensity by applying sufficient tension to a face part of a panel even though a straight safety band is used.
It is another object of the present invention to provide a panel in flat-type CRT, which is capable of effectively reducing an advance of a crack and a scattering of fragments due to external shock.
It is a further object of the present invention to provide a panel in flat-type CRT, which optimizes the relationship among an MMH (Mold Match Height), a CFT (Center Face Thickness) and an OAH (Overall Height), which are design factors, to make distribution of the stress of an outer surface of the CRT even and to prevent the concentration of tension stress.
To achieve the above objects, the present invention provides a structure of panel in flat-type CRT (Cathode Ray Tube), which includes a face part having a flat outer surface and an inner surface of a fixed curvature, and a skirt part extending from an edge of the face part to a rear portion, wherein, when a height from a MML (Mold Match Line) to an outer center of a face of the panel is designated as a MMH and a thickness of the center of the face surface of the panel is designated as a CFT, the relationship between the MMH and the CFT satisfies MMHxe2x89xa6CFT, the MML being an extension line of a match line between an upper external mold and a lower external mold to form the panel.
When a height from an end of the skirt part of the panel to a front surface of the face part is designated as an OAH, the relationship between the OAH and the CFT satisfies 0.12xe2x89xa6CFT/OAHxe2x89xa60.15.